Skeletal muscle atrophy is a poorly understood yet nearly universal consequence of severe human illness for which no therapy currently exists. We hypothesize that a central event in the pathogenesis of skeletal muscle atrophy is increased expression of ATF4, an evolutionarily ancient transcription factor that is induced by stress. This hypothesis is based on several lines of evidence derived from our preliminary studies. First, we found that ATF4 mRNA levels were increased in atrophied skeletal muscle from human subjects with spinal cord injuries. Second, we found that transfection of mouse skeletal muscle with plasmid DNA encoding mouse ATF4 induced myofiber atrophy. Conversely, transfection of an artificial microRNA targeting ATF4 or a dominant negative ATF4 construct reduced myofiber atrophy under fasting conditions. These results identify ATF4 as a novel transcriptional regulator of muscle mass that may be both necessary and sufficient for skeletal muscle atrophy. With the long-term goal of identifying new therapeutic approaches for muscle atrophy in human patients, we propose three aims. First, to determine if ATF4 is essential for muscle atrophy, we will study skeletal muscle-specific ATF4 gene knockout mice and determine if they are resistant to fasting- or denervation- induced muscle atrophy. Second, to determine the upstream mechanism of increased ATF4 expression, we will use RNA interference in wild-type mice and determine if one of the four mammalian stress-activated eIF2alpha kinases is required for ATF4 expression and muscle atrophy. Third, to determine the downstream mechanism(s) of ATF4-mediated atrophy, we will transfect skeletal myotubes with adenovirus expressing ATF4 and determine if this decreases protein synthesis and/or increases protein breakdown. We will also study skeletal muscle-specific ATF4 gene knockout mice to determine if ATF4 is required for induction of atrophy-associated proteins that inhibit protein synthesis (4E-BP1), increase protein synthesis (atrogin-1 and MuRF1), or if ATF4 activates amino acid transporter genes that are essential for muscle atrophy. PUBLIC HEALTH RELEVANCE: Skeletal muscle wasting is a very common medical condition for which no treatment currently exists. We hypothesize that a central cause of skeletal muscle wasting is a protein called ATF4, which acts as a switch that turns on genes for muscle wasting. By studying ATF4 and the genes it regulates, we hope to understand how muscle wasting occurs, and to identify new ways to treat patients with muscle wasting.